1. Field of the Invention
This invention relates in general to method and apparatus for transmitting encoded data.
2. Description of the Prior Art
It is known generally to synchronize ciphering equipment in a transmitting and a receiving station and to use codes for the encoding and decoding which can be operated synchronously. This is accomplished by using transmitting ciphering equipment-associated data from the transmitter to the receiving station. It is also known to transmit data from a transmitting station to a receiving station according to prescribed transmission procedures using data blocks. An example of such a procedure dependent data transmission is a system of transmission according to CCITT recommendation X.25 in a packet switching data network. This data network is known in the Federal Republic of Germany as DATEX-P. In such packet switching data networks the messages to be transmitted from the transmitting station to the receiving station are dissected into individual packets. Each packet receives its own heading with a control and an address field as well as its own check character sequence. The maximum packet length is 128 octets. The division of the message upon combination after transmission is carried out by the corresponding data terminal equipment itself. The data terminal equipment are connected to corresponding packet switching centers which are connected to each other. The respective received switching center checks each packet for transmission errors. Packets which are received error free are responded to with a positive acknowledgement. Packets which have errors are requested to be sent again by emitting a negative acknowledgement. Packets which have errors are requested to be sent again by emitting a negative acknowledgement. The individual packets can be transmitted in various ways. As a result, fast transmission and matching to the respective user status of the transmission links and of the packets switching centers is achieved.
Transmission can occur over selected connections or over permanently connected lines. When using selected connections, the data terminal equipment of the transmitting and receiving stations are only at times connected to each other over the data network. Thus, steps for the connection setup, data transmission and connection cleardown occur. When a dedicated connection is used, no connection setup and connection cleardown phase is required. The actual data transmission occurs in a manner similar to that for the selected connection. Both types of connection are virtual connections, in other words, the transmission can occur over different transmission links but appears to the user as though a single physical connection were present.
The interface to the data terminal equipment defined by CCITT recommendation X.25 is subdivided into three function levels with level one describing the setup of the connection path and the electrical and functional properties of the interface. Level two specifies the transmission procedure for the transmission of the packets between the data terminal equipment and the packet switching exchange. Level three determines the packet formats and the sequence of the packet transmission between the data terminal equipment and the packet switching exchange when a virtual call and a virtual dedicated circuit is utilized.
In case the data is encoded when transmitted it is necessary to transmit ciphering equipment associated data from ciphering equipment at the transmitting subscriber to the ciphering equipment at the receiving subscriber.